Anatomical insights into the proximal aponeurosis of the long head of the biceps femoris.

The biceps femoris long head (BFlh) is prone to strain injuries, but its reasons remain unclear. This study analyzed the BFlh proximal intramuscular aponeurosis in donor samples (n=4) through morphometric, microscopic, and histological methods. Cross-sections were taken every 5% of the muscle belly to differentiate connective, adipose, and muscle tissues. The aponeurosis extended from the muscle surface, becoming intramuscular from 40-70% of the muscle belly, and ended distally. Quantitative analysis revealed significant reductions of size in both the cross-sectional area (CSA) and width of the aponeurosis at 50% of muscle length, with CSA ranging from 4.9 mm² to 13.4 mm² and widths from 6.8 mm to 12.4 mm across subjects. Dense connective tissue bundles were separated by adipose or loose connective tissues. The aponeurosis shape varied along the muscle, with T- and hook-shaped configurations, and small branches were observed distally. These findings reveal the BFlh proximal aponeurosis as a complex structure, potentially influencing its injury susceptibility.